SkyEye

The Phases of the Moon

The appearance of the Moon is ever changing. Sometimes it is visible during the night and sometimes during the day. A thin sliver may be lit or the entire disk. It is normally silvery white but can appear blood red or even blue. What causes these changes?
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The accompanying diagram shows how the illuminated portion of the Moon changes shape as our satellite orbits around us. The phase is controlled by the Moon's position relative to Earth and the Sun. The inner circle shows the Moon in its orbit about Earth, with the lit portion always facing the Sun. The outer glyphs show how the Moon appears to observers on Earth. (North is up on these glyphs.)

The Moon always keeps the same face towards the Earth. This happens because the Moon rotates on its axis at the same angular speed as it orbits the Earth, a phenomenon known as synchronous rotation which is a special kind of tidal locking. Although the Moon is spherical, the hemisphere that faces the Earth has a much thinner crust than the far side. The Earth's gravity acts on this unequal distribution of mass, forcing the 'lighter' hemisphere to point always towards the Earth.

Telescopic observation of the Moon's limb reveals that we can actually see more than half of the surface, due to an effect called libration. This is a combination of a small oscillation of the body of Moon, called the physical libration, and the changing viewpoint of the Earth because the Moon's orbit is not perfectly circular, called the optical libration. Together, they bring approximately 59% of the Moon's surface into view from the Earth.

The remaining 41% of the Moon's surface remained invisible to humans until the Russian spacecraft Luna 3 sent back the first images in 1959. The lunar far side is utterly different to the side that we see from the Earth. There are only a few of the smooth, dark lava plains known as maria or 'seas' which dominate the near side. Instead, the far side is very heavily cratered.

New

When the Moon is in a line between the Sun and Earth, the unlit portion of our satellite faces us. New Moon marks the beginning of a lunation or lunar month.

Waxing Crescent

After the new phase, the Moon becomes visible as a crescent, trailing the Sun across the sky and seen during the day. The crescent gets a little fatter each day.

First Quarter

The Moon is half lit. It is visible in the sky during the afternoon and evening hours.

Waxing Gibbous

The phase continues to increase, with the Moon rising later and later in the day and visible after local midnight.

Full

The Moon is again in line with the Sun and Earth but this time our planet is between our star and our satellite. The Full Moon rises at sunset and sets at sunrise.

Waning Gibbous

During the waning phases, the Moon precedes the Sun through the sky. It can be seen rising later and later in the evening.

Last Quarter

Once again the Moon is half illuminated, rising around local midnight and visible during the morning until setting at approximately noon.

Waning Crescent

This phase is seen during the earlier parts of the day, with the Sun following behind. The crescent dwindles until the cycle begins with the new phase.

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The orbit of the Moon is tilted 5.1° with respect to the ecliptic (the apparent path of the Sun across the sky). As a result, the New Moon usually passes a little to the north or south of the Sun and the Full Moon similarly misses the shadow of the Earth. A node is where the Moon's orbit intersects the ecliptic. If the New Moon is very near to a node, then a solar eclipse may occur. Lunar eclipses only happen when the Full Moon is near or at a node. Because a totally eclipsed Full Moon often appears rusty red in colour, it is sometimes called a Blood Moon. During a total lunar eclipse, the only light reaching the Moon is sunlight that has passed through the Earth's atmosphere. The blue component of this light has been removed by Rayleigh scattering, which is also why sunsets are red. In effect, the eclipsed Moon is being lit by all of the sunsets in the world.

The Moon's orbit around Earth is not a circle but an ellipse. This means that the distance between Earth and the Moon is constantly varying, and so is the apparent angular diameter of our satellite. The nearest point in this orbit is called perigee and the farthest point apogee. When a Full Moon occurs near perigee, it appears slightly larger than usual and is sometimes referred to as a Super Moon. However, if a Full Moon takes place during apogee, it might be called a Micro Moon since its apparent diameter is slightly smaller than average. Note that the range in variation between these extremes and the average is only around 5.3% and thus not easy to detect with the naked eye.

Many people claim that the Moon appears larger when it is close to the horizon than when it is high in the sky. This is the well-known Moon Illusion. It has been suggested that when the Moon is near the horizon, the eye judges its size in comparison to nearby objects such as trees and buildings. When the Moon is high in the sky, however, the eye has nothing to compare it with, and as a result, the brain mis-judges its real size.

The Many Names of the Full Moon

Old almanacs often give the traditional names for northern hemisphere Full Moons but they are rarely used today with the possible exceptions of Harvest Moon, the Full Moon nearest to the northern hemisphere autumn equinox and usually (but not always!) falling in the month of September, and the Hunter's Moon, the next Full Moon after that. The Paschal Moon is the Full Moon before Easter. The following list is only approximate because a lunar month is shorter than a calendar month, causing the names to 'slide around' a little over the course of a year.

One way to keep the names aligned with the seasons is to insert an extra name once in awhile. If four rather than three Full Moons occur in a season, almanac editors call the third one a Blue Moon. A misunderstanding of this rule back in the 1940s led to a new definition of a Blue Moon, namely, the second Full Moon in a calendar month. For more information about all types of Blue Moons and the acclaimed Blue Moon Calculator, visit Once In a Blue Moon.